Although it is common to manufacture optical elements made of plastic using a molding process, recently molding processes particularly known as glass molding methods have been used for molding lenses made of glass. For example, in imaging optical systems used for digital still cameras and video cameras, because aspheric lenses have recently come to be widely used, such molding processes designed for mass production have been widely used for producing aspheric lenses.
Molding processes for forming glass lenses include a heating process to soften the glass used as the raw material, a pressing process to press mold the softened glass raw material within a mold, a cooling process to cool the molded glass after this press molding, and a removal process to remove the cooled molded product from the mold. As a result, the refractive index distribution inside the molded product, which should ideally be uniform, may become non-uniform. If an optical system is manufactured using this molded optical element of which the refractive index distribution has become non-uniform, aberrations occur due to the non-uniformity of the refractive index distribution, degrading the optical performance.
Japanese Laid-Open Patent Application 2000-249917 describes a method of correction which uses aspheric surfaces in order to correct aberrations caused by the non-uniformity of the refractive index distribution (non-uniformity of the refractive index distribution in the radial direction centered on the optical axis) inside a lens of a projection optical system or an illumination optical system used in an exposure device. However, Japanese Laid-Open Patent Application 2000-249917 states that polishing is used as part of the lens processing, and there is no description regarding the non-uniformity of the refractive index distribution due to molding.
Recently, image pickup elements such as CCDs (Charge Coupled Device) used in digital still cameras, including image pickup elements that include more than five million pixels have been developed, and the number of pixels will continue to increase. Therefore, higher and higher resolutions are demanded of imaging lenses used in digital still cameras and similar imaging devices. Consequently, non-uniformity of the refractive index distribution inside the lens due to molding is a problem that can no longer be ignored in determining optical performance.
There is no description in Japanese Laid-Open Patent Application 2000-249917 of the non-uniformity of the refractive index distribution due to molding, and it is believed that the non-uniformity of the refractive index distribution that is described therein does not consider the non-uniformity generated by the processing, but relates to the non-uniformity of the refractive index distribution of the lens material itself that is placed in the mold. Also, while an aspheric surface is used for correcting aberrations caused by the non-uniformity of the refractive index distribution, processing of the aspheric surface is performed by the further processing of a molded lens element. It is believed that this is performed, for example, by further polishing of the lens element. Because it is difficult to remold a molded lens element after molding, the method described in Japanese Laid-Open Patent Application 2000-249917 cannot be immediately adopted to an optical system having a molded optical element. Although it is believed that individually reprocessing each lens element which has been manufactured is effective for optical systems of exposure devices that are to be produced in small quantity and for which expensive production is acceptable, it is not suitable for optical systems for products that are to be mass produced, such as digital still cameras, because it is too labor intensive and too costly.
On the other hand, much more non-uniformity of the refractive index distribution due to molding is likely to occur for certain forms of molded lens elements, for example, especially in cases where the lens element has a concave optical surface where the difference in thickness between the center and the periphery is large. This is believed to be a result of the cooling process after press molding, wherein the speed of cooling and contraction is different in different parts of the lens element, and this difference generates internal distortions. Therefore, there are many cases where the non-uniformity of the refractive index distribution is similarly distributed for molded optical elements of the same form. Additionally, in manufacturing an optical system having a molded optical element, properties attributable to molding need to be considered. Note that methods of manufacturing optical elements that use a long time in the cooling process or in relieving non-uniformity of the refractive index distribution by an annealing process after molding are not preferred because they tend to increase production costs.